I. Field of the Invention
The present invention relates generally to metal treating methods and, more particularly, to a method for hot stamping iron based components.
II. Description of the Prior Art
There are many industrial applications in which a very hard component is required. For example, in automotive vehicles, the components such as the vertical pillars for the automotive vehicle are typically constructed of very hard materials to protect the occupants of the vehicle in the event of a crash.
One common hard material used in automotive applications is martensite, an allotrope of carbon steel. In order to form the martensite component, the sheet stock of carbon based steel is first heated to a temperature necessary to transform the sheet stock to austenite. Thereafter, the heated sheet stock is positioned within a stamping die and the die is closed to mechanically transform the bar stock to the desired end shape for the component. The now formed component is then quenched at a rate sufficient to transform the austenite to martensite while in the die. After quenching, the component is removed.
The component now transformed into martensite exhibits superior hardness and stiffness sufficient for the component to be used in applications where the hardness and stiffness is desired.
One difficulty, however, of martensite components is that it is difficult to perform post hot stamp operations such as trimming and piercing or the attachment of fastening nuts in such components. For example, conventional tool steel punches that are normally used to punch holes in steel components are inadequate for punching openings in martensite components.
Consequently, in order to form openings or attach fastening nuts within the martensite components, it has been previously necessary to utilize other methods, such as laser cutting, to trim and pierce the openings in the component. These alternative methods, however, are expensive compared to conventional punching methods, and thus increase the overall manufacturing cost of the component.
There have, however, been attempts to punch the required openings or attach a fastening nut in the component prior to transforming the component into martensite. However, due to the thermal expansion of the component while heating the component to a temperature sufficient to transform the steel to austenite, such “prepunching” of the openings in the component makes it difficult, if not altogether impossible, to accurately locate and size the openings in the component to the tolerances required by the automotive industry as well as other industries. Also this causes residual tensile stress which can lead to a potential delay fracture.